There's never a good time to get clobbered by an asteroid  something the dinosaurs discovered in the worst way possible. It was 65.5 million years ago when an asteroid measuring 6 miles (10 km) across slammed into the earth just off the coast of the Yucatán Peninsula, blasting out a 110-mile (180 km) crater and sending out a cloud of globe-girdling debris that cooled and darkened the world. That spelled doom for species that had come to like things bright and warm. Before long (in geological terms, at least) the dinos were gone and the mammals arose.

That's how the story has long been told, and it's still the most widely accepted theory. Now, however, a study led by scientists at the American Museum of Natural History in New York City and published in Nature Communications suggests that the asteroid might not have affected all dinosaur species equally. Some, including the well-loved triceratops and duck-billed dinosaurs, might have been on their way out already and were simply hastened to the exit by the asteroid blast. The reason for their weakened state  and the way the investigators discovered it  provides both new insights into the fate of the dinosaurs and new methods with which to study their world.

The asteroid impact  known as the Cretaceous-Tertiary (K-T) extinction  was always thought to have been an equal-opportunity annihilator, and there was good evidence to support that. Tracking the rise and fall of the dinosaurs was always done simply by counting how many species were around at any given moment in history. The more species there were, the better the overall clade was doing; the fewer there were  particularly after the K-T  the closer to extinction all dinosaurs came. But that method was never entirely reliable, mostly because paleontologists do their digging in so many different places.

"Results can be biased by uneven sampling of the fossil record," says Steve Brusatte, a graduate student at Columbia University and one of the participants in the new study. "In places where more rock and fossils were formed, like in America's Great Plains, you'll find more species." Similarly, in places that didn't fossilize remains easily, you'd find far fewer  even if at one time there were just as many animals there.

The Natural History team, led by paleontologist Mark Norell, thus decided to take a different approach  looking at the biodiversity within different groups of dinosaurs. If one group  the carnivores, say  was thriving, it ought to be producing more species than groups that were struggling just to hang on. When the investigators looked at things this way  sampling 150 species across seven major groups  they were able to paint a much different and much-less-uniform picture of how all the dinosaurs were faring before the asteroid arrived.

In general, the number of species in the small herbivore group (the ankylosaurs and pachycephalosaurs) was stable or even increasing. The same was true for the carnivores (the tyrannosaurs and coelurosaurs) as well as for the largest herbivores (the sauropods). Things were not so good for the slightly smaller herbivores known as bulk feeders because of the wide range of vegetation they ate (the hadrosaurs and ceratopsids). They appear to have been in decline for a good 12 million years before the K-T wipeout, with their species head count dwindling steadily over that time.

"People often think of the dinosaurs being monolithic," says Richard Butler of Ludwig Maximilian University in Munich, who also participated in the study. "We say, 'The dinosaurs did this, the dinosaurs did that.' But dinosaurs were hugely diverse. Different groups were probably evolving in different ways and the results of our study show that very clearly."

So why were the hadrosaurs and ceratopsids having such a hard time? Geography may explain at least some of the problems. The bulk feeders were especially common in North America, a continent that was then bisected by the Western Interior Seaway, a wide and deep body of water that ran from what is now the Arctic Ocean to what is now the Gulf of Mexico. Changes in the depth, width and temperature of the sea might have reduced the food supply or altered the surrounding ecosystem in other ways that made it hard for the hadrosaurs and ceratopsids to survive. The tectonic collisions, which gave rise to what are now the Rockies and the other mountains of the west, might have had a similar effect.

Whatever the cause of the two groups' decline, it's not certain that their condition was terminal  that they would not have somehow stabilized themselves if the asteroid hadn't come along and rendered the whole question academic. Indeed, throughout the whole of the Mesozoic Era  from 250 million to 65 million years ago  diversity within dinosaur species was known to fluctuate quite a bit. "Small increases or decreases between two or three time intervals may not be noteworthy within the context of the ... history of the [groups]," says Norell.

Of course, the asteroid did come along and did render everything academic. But if all of the dinosaurs left history's stage at more or less the same time and for more or less the same reason, they now appear to have strutted their hour in ways that were more varied  and in some cases more fraught  than we ever appreciated before.